1. Field of the Invention
The present invention relates to an acceleration sensor which detects acceleration in three-axis directions of an X-axis direction and a Y-axis direction and a Z-axis direction, all of which are perpendicular to each other.
2. Description of the Related Art
FIG. 15A illustrates a schematic perspective view of one example of an acceleration sensor (see, for example, Japanese Unexamined Patent Application Publication No. 2002-296293 (Patent Document 1)). The acceleration sensor 40 includes a frame portion 41, a cylindrical weight member 42 which is arranged in the center portion of the frame portion 41, X-axis direction beam portions 43a and 43b, each of which has an elongated shape in the X-axis direction from both sides in the X-axis direction of the weight member 42 to the frame portion 41, Y-axis direction beam portions 44a and 44b, each of which has an elongated shape in the Y-axis direction from both sides in the Y-axis direction of the weight member 42 to the frame portion 41, four reinforcing weight members 45a through 45d serially connected to the weight member 42, resistors Rx1 through Rx4 and Rz1 through Rz4 which are arranged on the X-axis direction beam portions 43a and 43b, and resistors Ry1 through Ry4 which are arranged on the Y-axis direction beam portions 44a and 44b. 
With the configuration of the acceleration sensor 40 illustrated in FIG. 15A, the central axis of the X-axis direction beam portions 43a and 43b are arranged on the same straight line extending along the X-axis direction passing through the central axis of the cylindrical weight member 42, and further, the central axis of the Y-axis direction beam portions 44a and 44b are arranged on the same straight line extending along the Y-axis direction passing through the central axis of the cylindrical weight member 42. These X-axis direction beam portions 43a and 43b and Y-axis direction beam portions 44a and 44b are configured so as to be capable of bending deformation.
The resistors Rx1 and Rx2 are arranged along the X-axis direction on the X-axis direction beam portion 43a, and the resistors Rx3 and Rx4 are arranged along the X-axis direction on the X-axis direction beam portion 43b. The resistors Ry1 and Ry2 are arranged along the Y-axis direction on the Y-axis direction beam portion 44a, and the resistors Ry3 and Ry4 are arranged along the Y-axis direction on the Y-axis direction beam portion 44b. Resistors Rz1 and Rz2 are arranged along the X-axis direction on the X-axis direction beam portion 43a, and resistors Rz3 and Rz4 are arranged along the X-axis direction on the X-axis direction beam portion 43b. These resistors Rx1 through Rx4, Ry1 through Ry4, and Rz1 through Rz4 each can have the electrical resistance value thereof changed by the stress change of the beam portions 43a, 43b, 44a, and 44b from the bending deformation of the beam portions 43a, 43b, 44a, and 44b. 
Wiring is provided on each of the beam portions 43a, 43b, 44a, and 44b, and the frame portion 41 in order for the four resistors Rx1 through Rx4 to define a bridge circuit such as that shown in FIG. 15B, and in order for the four resistors Ry1 through Ry4 to define a bridge circuit such as that shown in FIG. 15C, and in order for the four resistors Rz1 through Rz4 to define a bridge circuit such as that shown in FIG. 1D. It should be noted that the reference symbol Vcc shown in FIGS. 15B through 15D shows a voltage power input portion which is connected to an external voltage power source, and the reference symbols Px1, Px2, Py1, Py2, Pz1, Pz2 each show a voltage detecting portion.
The weight member 42 and the reinforcing weight members 45a through 45d are each in a floating state, and are capable of being displaced by the bending deformation of the beam portions 43a, 43b, 44a, and 44b. For example, when the force in the X-axis direction originating from the acceleration in the X-axis direction acts against the weight member 42 and the reinforcing weight members 45a through 45d, the weight member 42 and the reinforcing weight members 45a through 45d swing and are displaced in the X-axis direction. Similarly, when the force in the Y-axis direction originating from the acceleration in the Y-axis direction acts against the weight member 42 and the reinforcing weight members 45a through 45d, the weight member 42 and the reinforcing weight members 45a through 45d swing and are displaced in the Y-axis direction. Further similarly, when the force in the Z-axis direction originating from the acceleration in the Z-axis direction acts against the weight member 42 and the reinforcing weight members 45a through 45d, the weight member 42 and the reinforcing weight members 45a through 45d swing and are displaced in the Z-axis direction. The beam portions 43a, 43b, 44a, and 44b are subjected to bending deformation by such displacement of the weight member 42 and the reinforcing weight members 45a through 45d. 
With the acceleration sensor 40, the resistance value of the resistors Rx1 through Rx4, Ry1 through Ry4, and Rz1 through Rz4 varies by the stress generated by the beam portions 43a, 43b, 44a, and 44b from the bending deformation of the beam portions 43a, 43b, 44a, and 44b. From the change in resistance value of the resistors, the resistance value of the four resistors of each bridge circuit in FIGS. 15B through 15D becomes out of balance. For example, when acceleration is occurring in the X-axis direction, this results in differences in the voltage output by each of the voltage detecting portions Px1 and Px2 of the bridge circuit in FIG. 15B. Using this voltage difference, the amount of acceleration in the X-axis direction can be detected. Also, when acceleration is occurring in the Y-axis direction, this results in differences in the voltage output by each of the voltage detecting portions Py1 and Py2 of the bridge circuit in FIG. 15C. Using this voltage difference, the amount of acceleration in the Y-axis direction can be detected. Further, when acceleration is occurring in the Z-axis direction, this results in differences in the voltage output by each of the voltage detecting portions Pz1 and Pz2 of the bridge circuit in FIG. 15D. Using this voltage difference, the amount of acceleration in the Z-axis direction can be detected.
With the configuration of the acceleration sensor 40 illustrated in FIG. 15A, the beam portions 43a, 43b, 44a, and 44b which are linear are each arranged on four sides of the weight member 42, and link the weight member 42 to the frame portion 41. Thus, if distortion occurs to the frame portion 41 due to thermal stress, the distortion occurs to the beam portions 43a, 43b, 44a, and 44b corresponding to the distortion to the frame portion 41, compression stress or tension stress occurs to the beam portions 43a, 43b, 44a, and 44b. The resistors Rx1 through Rx4, Ry1 through Ry4, and Rz1 through Rz4 for detecting acceleration are provided on the beam portions 43a, 43b, 44a, and 44b, and therefore, regardless of acceleration not occurring, the electrical resistance values of the resistors Rx1 through Rx4, Ry1 through Ry4, and Rz1 through Rz4 change by the stresses generated on the beam portions 43a, 43b, 44a, and 44b, due to distortion caused by the thermal stress of the frame portion 41. Thus, regardless of acceleration not occurring, voltage can be output from the bridge circuits in FIGS. 15B through 15D when acceleration is generated.
With the configuration of the acceleration sensor 40 illustrated in FIG. 15A, the acceleration-detecting resistors Rx1 through Rx4, Ry1 through Ry4, and Rz1 through Rz4 are provided on the beam portions 43a, 43b, 44a, and 44b which are arranged on the four sides of the weight member 42, and the arrangement positions of the resistors are scattered. For example, in the event that the beam portions 43a, 43b, 44a, and 44b are configured with silicon, the resistors Rx1 through Rx4, Ry1 through Ry4, and Rz1 through Rz4 which are doped with phosphorus (P) or boron (B) and are piezoresistant are provided on the resistor arrangement positions on the beam portions 43a, 43b, 44a, and 44b. In this event, if the resistor arrangement positions are scattered, uniform doping with phosphorus or boron at the respective resistor arrangement positions is difficult, and the doping concentration varies at the respective resistor arrangement positions. Thus, balancing the resistance value of the four resistors on each bridge circuit illustrated in FIGS. 15B through 15D is difficult, and a problem occurs wherein improvements in the precision of detecting acceleration are prevented.
Other examples of known acceleration sensors are disclosed in Japanese Unexamined Patent Application Publication No. 8-160070 (Patent Document 2) and Japanese Unexamined Patent Application Publication No. 6-82472 (Patent Document 3).